Aluminum alloys are now mainly used in the manufacture of heat exchangers for cars because of their low density, which provides a saving in weight, in particular when compared to copper alloys, while providing good thermal conductivity, being easy to use and having good resistance to corrosion.
All aluminum alloys discussed in this application are designated, unless otherwise stated, according to the designations defined by the ANSI H35.1/H35.1(M)-2006 standard, Revision of H35.1H35.1(M)-2004, American National Standard Alloy and Temper Designation System for Aluminum, published by The Aluminum Association, Inc.
Exchangers comprise tubes for the circulation of the internal heating or cooling fluid and fins to increase heat transfer between the internal fluid and the external fluid, and they are manufactured either by mechanical assembly, or by brazing.
In the more frequent case of assembly by brazing, the core sheet constituting the tube, generally made of aluminum alloy of the AA3xxx series, is clad on the outside, in contact with the fins, with a so-called brazing alloy, generally of the AA4xxx series. This configuration is illustrated in FIG. 1, diagram 1a, the core sheet having the reference mark 2 and the brazing alloy reference mark 1.
The latter melts at a temperature lower than the melting point of the core and, by applying a thermal brazing cycle, is able to create a connection between the two materials to be brazed: the fins and the outside of the tube.
The core sheet may also be clad on the inside by a layer of protection 3 against corrosion and erosion by the coolant or heat transfer fluid. This configuration is illustrated in FIG. 1, diagram 1b. 
This latter layer, generally also in the form of co-rolled sheet, is known in the profession as the “inner-liner”; it is generally made of an alloy of the AA7xxx series.
The internal cladding alloy most frequently used to date is of type AA7072.
Because of its relatively large zinc content, on average 1.05% by weight expressed as a percentage, its corrosion potential is lower than that of the core, typically made of alloy of the Al—Mn—Cu type, which enables it to perform its role of sacrificial anode.
As examples of Al—Mn—Cu core alloys, mention may be made of alloys 3916 and 3915 described respectively in patent EP 1075935 and request EP 1413427 by the applicant; their compositions are given below as percentages by weight, not counting impurities limited to 0.05% each and 0.15% in total
SiFeCuMnMgZnTi39160.15-0.30<0.250.5-1.01.0-1.4<0.01<0.2<0.139150.15-0.30<0.250.5-1.01.0-1.40.10-0.35<0.2<0.1
However, because of its solidus temperature, of the same order as that of core alloys in regularly use, i.e. approximately 640° C., alloy AA7072 does not take part in brazing.
In addition, for reasons of heat exchange efficiency, tubes with a so-called B section, as shown in FIG. 2 are replacing simple welded rolled tubes more and more.
They are obtained by folding, in particular from a brazing strip or plate such as previously defined, i.e. with a brazing cladding on their outside, and a sacrificial cladding on the inside.
However, brazing of this type of tube is difficult, particularly at the level of the outside of foot 6 as shown inside the circle of FIG. 2. seen from the left.
Obtaining a correct brazing joint 5 in this zone requires a significant amount of brazing alloy 4 from cladding 1 to be contributed, while the available external volume of said brazing alloy is limited and located only at the level of the center of the foot as illustrated in FIG. 2 viewed from the left, in the center of the circle, and in diagrams 1a and 1b. 
One of the known solutions to this problem involves increasing the thickness of external cladding 1 to give a greater addition of metal in zone 6.
It should be pointed out that, typically, in the case of a roll-welded tube which is not subject to this problem, external cladding corresponds to 10% of the total thickness of the brazing sheet as against 10% for the internal cladding, for a total thickness generally ranging between 200 and 300 μm.
In the case of a folded and brazed tube with a B section, in order to increase the thickness of the external cladding 1 of brazing alloy, it is necessary either to increase the total thickness of the brazing sheet by preserving the above-mentioned percentages, or to increase the percentage of external cladding with a constant thickness of brazing sheet.
These two solutions are clearly not satisfactory.
The first goes completely against the general trend in the automobile field, and more particularly in that of heat exchangers, which involves reducing the thickness of components as much as possible.
The second involves reducing the thickness of the core which is precisely what mainly provides the mechanical resistance and the corrosion resistance of the material.
Another known solution involves using a brazing alloy of type AA4045 or AA4343 for the internal cladding or “inner-liner”. But such an alternative inevitably results in an unacceptable drop in the corrosion resistance, in particular as measured by the test referred to as “OY” of experts in the field and described below.
Lastly, requests JP2005037062 by Toyo Radiator, JP2004217982 and JP2004217983 by Sumitomo Light Metal describe alternative solutions primarily consisting in folds of brazing sheet in the central zone of the foot so as to bring the two parts of the external cladding into contact and thereby guarantee brazing.
Such solutions, just as the first one described, have in particular the disadvantage of inducing a surplus of material used and an increase in overall spatial requirements for a constant internal fluid circulation section.